Current supply circuits for electrical motors, for example to supply current from a battery to an electrically driven fork lift loader or the like, to electric vehicles such as run-abouts, and electrically driven automobiles, use circuits in which the energy supplied by the battery is most efficiently applied to the motor. In a frequently used circuit, a thyristor is pulsed in accordance with desired operating conditions of the motor to transfer only that amount of energy from the battery as can be actually used under the desired conditions. The operation of the thyristor must be carefully monitored and various control arrangements, including supervisory monitoring arrangements, have been proposed. If the main current controlling thyristor should become defective, for example burn out and form a short circuit, dangerous operating conditions might result. Likewise, the control unit may become defective and gate the thyristor continuously ON. It is, therefore, necessary to supervise the motor current or motor voltage to prevent draining the supply battery, which typically is a secondary or storage battery. British Pat. No. 1,344,614 describes (to which U.S. Pat. No. 3,689,818, Morton et., corresponds) a control arrangement in which a motor current signal is provided which is compared with a control pulse signal which controls firing of the main thyristor. Circuits are connected to interrupt current supply to the main thyristor if a motor current signal is sensed although the control signal is absent or OFF.
Sensing of the motor current signal, although theoretically simple, causes difficulties in actual practice. If a measuring resistor is included in the main current supply circuit to the motor, non-linearities will result; direct current sensing can be used employing Hall generators which, however, are subject to output variations upon change in temperature. It has also been proposed to supervise the operation of the motor-thyristor control system by supervising motor voltage. Inductive voltages arising in the motor, which change with motor operation, can provide erroneous output signals which either may be in the form of "false alarms" or insufficient indication of malfunction.